Field of the Invention
The present invention relates to the technical field of magnetic resonance imaging, and particularly to a gradient coil.
Description of the Prior Art
Magnetic resonance imaging (MRI) is a technology in which the phenomenon of magnetic resonance is utilized for the purpose of imaging. The basic principles of magnetic resonance are as follows: when an atomic nucleus contains a single proton, as is the case with the nuclei of the hydrogen atoms that are present throughout the human body, this proton exhibits spin motion and resembles a small magnet. The spin axes of these small magnets lack an adhesive pattern, but when an external magnetic field is applied, the small magnets will be rearranged according to the magnetic force lines of the external magnetic field. Specifically, they will align in two directions, either parallel or anti-parallel to the magnetic force lines of the external magnetic field. The direction parallel to the magnetic force lines of the external magnetic field is called the positive longitudinal axis, while the direction anti-parallel to the magnetic force lines of the external magnetic field is called the negative longitudinal axis. The atomic nuclei only have a longitudinal magnetization component, which has both a direction and a magnitude. A radio frequency (RF) pulse of a specific frequency is used to excite the atomic nuclei in the external magnetic field such that their spin axes deviate from the positive longitudinal axis or negative longitudinal axis, giving rise to resonance—this is the phenomenon of magnetic resonance. Once the spin axes of the excited atomic nuclei have deviated from the positive or negative longitudinal axis the atomic nuclei have a transverse magnetization component.
Once emission of the RF pulse has ended, the excited atomic nuclei emit an echo signal, gradually releasing the absorbed energy in the form of electromagnetic waves, such that their phase and energy level both return to the pre-excitation state. An image can be reconstructed by subjecting the echo signal emitted by atomic nuclei to further processing, such as spatial encoding.
FIG. 6 is a schematic illustration in a radial cross section, of a gradient coil of a magnetic resonance imaging system according to the prior art. The gradient coil 500 is an important assembly in a magnetic resonance imaging (MRI) system. Generally, as shown in FIG. 6, the gradient coil has two coil sets: a primary coil set 501 for generating a positive gradient magnetic field, and a secondary coil set 502 for generating a negative gradient magnetic field. The primary coil set 501 is formed in a cylindrical shape; and the secondary coil set 502 is cylindrically sheathed on an outer periphery of the primary coil set. The secondary coil set is used to shield an eddy current from the primary coil set to a magnet. Each of the primary coil set 501 and the secondary coil set 502 comprises X, Y and Z layers of coils.
The magnetic resonance system has, between the primary coil set and the secondary coil set, a number of shimming slots 503 that are disposed between the primary coil set 501 and the secondary coil set 502 in a circumferential direction of the gradient coil.
The shimming slots are manufactured as follows. A shimming strip mold is provided for supporting the secondary coil set during the assembly. The shimming strip mold is fixed between the primary coil set and the secondary coil set in an axial direction of the gradient coil with two ends of the shimming strip mold. Epoxy resin is poured between the primary coil set and the secondary coil set; and after pouring, the shimming strip mold is ejected out of the whole gradient coil, so as to form shimming slots.
During the manufacture of the shimming slots, the shimming strip mold may possibly be distorted and deformed under the huge force of the secondary coil set, and when the shimming strip mold is ejected out, the epoxy resin may be ruptured due to deformation.